Mach Zehnder photonic crystal sensors and methods

ABSTRACT

A sensor apparatus includes a photonic crystal structure including a Mach Zehnder interferometer with a reference arm and a sensor arm configured to provide an evanescent field through a sensed medium region adjacent to the sensor arm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.10/951,916 entitled “Photonic Crystal Laser Sensors and Methods” filedherewith.

TECHNICAL FIELD

The present invention relates generally to sensors, and, moreparticularly, to the use of photonic crystal waveguides in sensors.

BACKGROUND ART

One form of chemical detection device employs spectroscopic-basedtechniques. For example, such a device samples air by passing it througha filter having a surface coating adapted to adhere to the chemicalvapors being detected. The filter traps molecules of the chemical vaporbeing detected and is then burned (i.e., vaporized) to produce a lightspectrum indicative of the presence or absence of the chemical vaporbeing detected. A spectrometer is then employed to split the variouswavelength components of the light spectrum due to the vaporization ofthe chemical vapor. The spectrometer produces a pattern of linescharacteristic of the presence or absence of the chemical beingdetected. The mass spectroscopic-based systems available today, however,tend to be too large and require too much power to be field portable.

Another type of chemical detection device employs quartz crystals asmechanical oscillators. Such devices generally measure the change infrequency of an oscillating quartz crystal as it is affected by the massof molecules which are being detected. The change in mass, however, ofquartz crystal oscillators as they absorb chemical vapors, is so smallthat the change in their frequency of oscillation is also extremelysmall. This limits the sensitivity of quartz crystal-based detectiondevices and the number of different applications in which they can bereliably employed.

It would be useful to be able to provide a sensing technology that ishighly sensitive, power efficient, and compact in size (e.g., nanometerscale).

DISCLOSURE OF INVENTION

According to an example embodiment, a sensor apparatus includes aphotonic crystal structure including a Mach Zehnder interferometer witha reference arm and a sensor arm configured to provide an evanescentfield through a sensed medium region adjacent to the sensor arm.

According to an example embodiment, a sensor apparatus includes multiplesensor elements, each of the sensor elements including a photoniccrystal Mach Zehnder interferometer with a reference arm and a sensorarm configured to provide an evanescent field through a sensed mediumregion adjacent to the sensor arm, and a mechanism for detecting outputsof the multiple sensor elements and converting the outputs into data.

According to an example embodiment, a method for sensing includesproviding a photonic crystal Mach Zehnder interferometer that generatesan evanescent field through a sensed medium region, providing an articleof medium at the sensed medium region, optically coupling a light sourceto an input of the photonic crystal Mach Zehnder interferometer, anddetecting an output of the photonic crystal Mach Zehnder interferometer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a ridge semiconductor laser;

FIG. 2 illustrates partial removal of the ridge from the laser of FIG. 1such that light is no longer being confined under the ridge and goesinto a slab mode;

FIG. 3 illustrates the formation of photonic crystal voids on thesemiconductor laser surface of FIG. 2 to guide the light;

FIG. 4A is a top view of an example embodiment of a laser sensorapparatus with a photonic crystal mirror;

FIGS. 4B and 4C are cross-sectional views of the laser sensor apparatuswith a photonic crystal mirror of FIG. 4A;

FIG. 5 is a top view of an example laser sensor apparatus with aphotonic crystal double pass interferometer that includes a sensor armand a passivated reference arm;

FIG. 6 is a top view of the laser sensor apparatus with a photoniccrystal double pass interferometer of FIG. 5 provided with a phaseshifter;

FIG. 7 is a top view of an example photonic crystal waveguide configuredfor combining the outputs of a sensor laser and a reference laser;

FIG. 8 is a top view of an example Mach Zehnder planar photonic crystalwaveguide sensor;

FIG. 9 is a diagram of an example sensor apparatus with multiple sensorelements; and

FIG. 10 illustrates an example evanescent field.

BEST MODES FOR CARRYING OUT THE INVENTION Definitions

Micron-scale dimensions refers to dimensions that range from 1micrometer to a few micrometers in size.

Sub-micron scale dimensions refers to dimensions that range from 1micrometer down to 0.05 micrometers.

Nanometer scale dimensions refers to dimensions that range from 0.1nanometers to 50 nanometers (0.05 micrometers).

PRESENT EMBODIMENTS

Various embodiments of the present invention pertain to interferometricand other detector structures that use photonic crystals for chemical,biological and/or other types of sensing with evanescent fields.

Evanescent fields are created by total internal reflection. FIG. 10illustrates an example of an exponentially decaying evanescent field1000 on the opposite side of a totally internally reflecting interface1002 (e.g., a waveguide). When a medium of high refractive index isbrought into the evanescent field, this frustrates the total internalreflection of the incident light 1004, changing the amplitude of thereflected light 1006. Sensors and sensing methods described hereinexploit evanescent fields associated with photonic crystal structures.

Referring to FIG. 1, an example ridge semiconductor laser 100 formed asshown includes an active layer 102, an overclad layer 104, an undercladlayer 106, and a ridge 108. Light output (along a direction denoted byarrow 110) from the ridge semiconductor laser 100 is guided by the ridge108 as indicated by ellipse 112. According to various embodiments andreferring to FIG. 2, the ridge semiconductor laser 100 is modified topartially remove the ridge 108 in region 114 as shown such that thelight is no longer confined under the ridge 108 and goes into slab modeas indicated by ellipse 116. Next, and referring to FIG. 3, photoniccrystal voids 118 are formed on the semiconductor laser surface suchthat the light is guided as indicated by ellipse 120, thereby providinga photonic crystal laser structure. In this example, the overclad layer104 and the underclad layer 106 have refractive indices lower than thatof the core layer 102, and all three layers are patterned with thephotonic crystal structure. The photonic crystal structure defines adefect waveguide. An optical signal traveling in the defect waveguide isconfined in the horizontal direction by the photonic crystal structure,and in the vertical direction by the lower refractive index claddinglayers. Changes in parameters of the photonic crystal will affect thephotonic bandgap of the photonic crystal, the band of allowed guidedmodes of the defect waveguide, and the propagation of an optical signalin the defect waveguide.

By way of example, and referring to FIGS. 4A-4C, such photonic crystalstructures can be used to provide an evanescent field through a sensedmedium region such that the photonic crystal structure functions as acavity/resonator for the laser. In this example embodiment, a lasersensor apparatus 400 includes a laser 402 and a photonic crystal mirrorstructure 404 which is optically coupled to the laser 402 as shown. Inthis example, the laser 402 is a semiconductor laser that includes anactive layer 406, an overclad layer 408, an underclad layer 410, and aridge 412; however, it should be appreciated that the principlesdescribed herein are applicable to other light sources (e.g., fiberlasers). The photonic crystal mirror structure 404 in this exampleembodiment includes photonic crystal voids 414 formed in a pattern asshown through the overclad layer 408, the active layer 406, and theunderclad layer 410. The photonic crystal mirror structure 404 is alsoprovided with a sensed medium region 416 (shown in dashed lines)positioned over the photonic crystal waveguide defined by the voids 414.In operation, a chemical, biological or other medium is placed in thesensed medium region 416. The evanescent field resulting from lightpropagating along the photonic crystal structure “probes” the medium.More specifically, the evanescent tail of the mode propagating along thephotonic crystal waveguide structure passes through the medium, and theresulting interactions with the medium can alter the propagation speedand/or attenuation of the evanescent tail. The thickness of the overcladlayer 408 can also be adjusted to provide a receptacle for the medium,to accommodate the refractive indices of various combinations of coreand cladding materials, etc. In this example embodiment, the voids 414are arranged in a pattern that provides the photonic crystal mirrorstructure 404. Thus, interaction between the evanescent field and themedium in the sensed medium region 416 effects the characteristics ofthe light (denoted by arrow 418) reflected by the photonic crystalmirror structure 404, thereby providing an output indicative of thesensed medium.

With respect to materials, the photonic crystal structures (e.g.,nanostructures and sub-micron structures) can be fabricated on III-Vsemiconductor materials (e.g., GaAs or InP and their alloys). Molecularbeam epitaxy (MBE) can be used to fabricate very thin layers for theIII-V semiconductors with a very accurate control during epitaxialgrowth.

Other materials can be used to fabricate the planar photonic crystalwaveguides described herein. Generally, the bulk materials can be anymaterial substantially transparent to the wavelengths of the opticalsignal. For example, the planar photonic crystal bulk material can bedoped silica, undoped silica, silicon, a polymeric organic material, anorganic/inorganic hybrid material, an inorganic glass (e.g.,chalcogenide glass), or any other suitable materials. The difference inrefractive index between the core and the cladding layers can beachieved by using two substantially different materials, or byselectively doping similar materials, or by other methods known to thoseskilled in the art. The voids can be filled with air, or with anothermaterial. In various embodiments, the material of the voids has arefractive index that is substantially different than the bulk photoniccrystal material. The geometry of the pattern of voids (more generally,the “photonic crystal structures”) can be hexagonal, square, triangular,rectangular, or otherwise, depending on the in-plane photonic band gapdesired. Moreover, the voids can be formed with shapes other thancylindrical.

The photonic crystal structures described herein can be fabricated in avariety of different ways. Nanoimprinting, a technique using nanoscaleto sub micron and micron scale patterns to stamp or print designs onchip surfaces, can be used. By way of example, a nanoimprintingtechnique can involve using a hard mold to create nanoscale features bydirectly imprinting into a polymer film. After a pattern has beenimprinted, the photonic crystal defects are created by etching thepattern (e.g., anisotropic dry etching with reactive ions).

Other photonic crystal structure fabrication techniques can be employed.For example, Focused Ion Beam (FIB) can be used to drill the photoniccrystal holes. To address any damage to optical/electrical qualitycaused by FIB, additional optical pumping and/or electrical can beprovided on the photonic crystal part to recover losses. Ultraviolet(UV) laser lithography, laser interference lithography, andelectron-beam lithography can also be used.

Photonic crystal mirrors can be used singly or, as described below, inan interferometric arrangement in a sensor apparatus. By way of example,and referring to FIG. 5, a laser sensor apparatus 500 includes a laser502 (e.g., a semiconductor laser) and a photonic crystal double passinterferometer structure 504 which is optically coupled to the laser 502as shown. In this example, the photonic crystal double passinterferometer structure 504 includes a sensor arm 506 and a referencearm 508 in a Y-configuration as shown. At the end of each arm, aphotonic crystal mirror is provided. In this example, the reference arm508 is passivated, as indicated by passivation region 510. The photoniccrystal double pass interferometer structure 504 is also provided with asensed medium region 512 (shown in dashed lines) positioned over thephotonic crystal waveguide of the sensor arm 506. In operation, achemical, biological or other medium is placed in the sensed mediumregion 512. The evanescent field resulting from light propagating alongthe photonic crystal structure “probes” the medium. More specifically,the evanescent tail of the mode propagating along the photonic crystalwaveguide structure passes through the medium, and the resultinginteractions with the medium can alter the propagation speed and/orattenuation of the evanescent tail. In this example, the thickness ofthe overclad layer 514 (of the laser 502) can also be adjusted toprovide a receptacle for the medium, to accommodate the refractiveindices of various combinations of core and cladding materials, etc. Inthis example embodiment, voids 516 are arranged in a pattern thatprovides the photonic crystal double pass interferometer structure 504.Thus, interaction between the evanescent field and the medium in thesensed medium region 512 effects the characteristics of the light(denoted by arrow 518) reflected by the photonic crystal double passinterferometer structure 504, thereby providing an output indicative ofthe sensed medium.

Referring to FIG. 6, in another embodiment, the reference arm 508 of anotherwise identical laser sensor apparatus 500′ is provided with a phaseshifter 520 for adjusting the operating point of the laser at an optimalor desired sensitivity point for detection (e.g., of a particular typeor species of medium). The phase shifter 520 can also be used forbiasing out manufacturing defects, compensating for contamination, andresetting the laser sensor apparatus 500′ to a new operating point.Phase shifting can be accomplished in semiconductors by carrierinjection which changes the refractive index of the semiconductorlocally. Carrier injection can be either electrical using pn junctionsor metal semiconductor junctions or optical pumping by injectingelectrons and holes of a local region.

Other sensor apparatuses include photonic crystal waveguide combinerstructures. By way of example, and referring to FIG. 7, a laser sensorapparatus 700 includes a sensor laser 702 and a reference laser 704(e.g., semiconductor lasers) and a photonic crystal heterodyningstructure 706 which is optically coupled to the sensor laser 702 and thereference laser 704 as shown. The sensor laser 702 is also provided witha sensed medium region 708 (shown in dashed lines) which is positionedover a photonic crystal structure 710 of the sensor laser 702. Inoperation, a chemical, biological or other medium is placed in thesensed medium region 708. The evanescent field resulting from lightpropagating along the photonic crystal structure 710 “probes” themedium. More specifically, the evanescent tail of the mode propagatingalong the photonic crystal structure 710 passes through the medium, andthe resulting interactions with the medium can alter the propagationspeed and/or attenuation of the evanescent tail. In this exampleembodiment, voids 712 are arranged in a pattern that provides thephotonic crystal heterodyning structure 706 (which combines the lightoutputs of the sensor laser 702 and the reference laser 704). In thisexample, the photonic crystal heterodyning structure 706 includesphotonic crystal partial mirrors 714 adjacent to the sensor laser 702and the reference laser 704 as shown. Thus, interaction between theevanescent field and the medium in the sensed medium region 708 effectsthe characteristics of the light (denoted by arrow 716) output by thephotonic crystal heterodyning structure 706, thereby providing an outputindicative of the sensed medium.

Photonic crystal structures as described herein can also be used toprovide passive sensor apparatuses. By way of example, and referring toFIG. 8, a Mach Zehnder planar photonic crystal waveguide sensor 800includes a photonic crystal Mach Zehnder interferometer structure 802and conventional waveguide-to-photonic crystal transition elements 804and 806 which are optically coupled as shown to the input and the outputof the photonic crystal Mach Zehnder interferometer structure 802,respectively. In this example embodiment, the photonic crystal MachZehnder interferometer structure 802 includes voids 808 arranged in apattern defining a sensor waveguide arm 810 and a reference waveguidearm 812 as shown. The photonic crystal Mach Zehnder interferometerstructure 802 is also provided with a sensed medium region 814 (shown indashed lines) positioned over the sensor waveguide arm 810. In thisexample, the reference waveguide arm 812 is provided with a phaseshifter 816 for biasing out manufacturing defects, compensating forcontamination, and resetting the Mach Zehnder planar photonic crystalwaveguide sensor 800 to a new operating point. Another method of phaseshifting in addition to carrier injection either optically orelectrically is to apply a DC bias and use the electro-optic effect inIII-V semiconductors. A field is applied via a metal semiconductor ormetal oxide semiconductor contact to the semiconducting region. In thisexample, optical fibers 818 and 820 are optically coupled to thetransition elements 804 and 806, respectively. In operation, a chemical,biological or other medium is placed in the sensed medium region 814.The evanescent field resulting from light propagating along the sensorwaveguide arm 810 “probes” the medium. More specifically, the evanescenttail of the mode propagating along the sensor waveguide arm 810 passesthrough the medium, and the resulting interactions with the medium canalter the propagation speed and/or attenuation of the evanescent tail.By varying the optical path length of the sensor waveguide arm 810, thedifference in optical path length between the sensor waveguide arm 810and the reference waveguide arm 812 controls the interference of theoptical signals propagating in those waveguides upon recombination.Thus, interaction between the evanescent field and the medium in thesensed medium region 814 effects the characteristics of the light(denoted by arrow 822) output by the photonic crystal Mach Zehnderinterferometer structure 802, thereby providing an output indicative ofthe sensed medium.

Sensor apparatuses including multiple sensor elements can also beprovided. By way of example, and referring to FIG. 9, a sensor apparatus900 (shown in dashed lines) includes multiple sensor elements 902,multiple photodetectors 904, a memory device 906, a processor 908 and atransponder 910, configured as shown. Each of the sensor elements 902 isconfigured to operatively interface with a sample of sensed medium 912(shown in dashed lines) as, for example, with the previously describedsensors. For example, each of the sensor elements 902 can be a lasersensor apparatus with a photonic crystal structure that defines amirror, a double pass interferometer, or a combiner, or a passive sensorapparatus with a photonic crystal Mach Zehnder interferometer or otherphotonic crystal structure. In operation, outputs of the sensor elements902 are detected by their corresponding photodectors 904, which convertthe detected light outputs to data signals that are provided to thememory device 906 and/or processor 908. By way of example, a signalanalysis computer-executable program and characteristic informationassociated with various media are stored in the memory device 906. Thedata signals are received and processed by the processor 908 whichexecutes the signal analysis program to provide processed data to thetransponder 910 for transmission. Alternatively, the signal analysisfunctionality can be implemented “off sensor”. In an example embodiment,the transponder 910 is a radio frequency identification (RF-ID)transponder configured to transmit data corresponding to the outputs ofthe sensor elements 902 in response to a sensor interrogation signal. Asillustrated in this example embodiment, such an interrogation signal canbe provided by a RF-ID reader 914.

Although the present invention has been described in terms of theexample embodiments above, numerous modifications and/or additions tothe above-described embodiments would be readily apparent to one skilledin the art. It is intended that the scope of the present inventionextends to all such modifications and/or additions.

1. A sensor apparatus comprising: a photonic crystal structure includinga Mach Zehnder interferometer with a reference arm and a sensor armconfigured to provide an evanescent field through a sensed medium regionadjacent to the sensor arm, the sensed medium region being part of thephotonic crystal structure.
 2. The sensor apparatus of claim 1, whereinthe Mach Zehnder interferometer is passive.
 3. The sensor apparatus ofclaim 1, wherein the sensed medium region is configured to receive achemical or biological medium.
 4. The sensor apparatus of claim 1,further comprising: a phase shifter for the reference arm.
 5. The sensorapparatus of claim 1, further comprising: means for optically couplingthe photonic crystal structure to an optical fiber.
 6. The sensorapparatus of claim 1, wherein the photonic crystal structure is formedfrom III-V semiconductor materials.
 7. The sensor apparatus of claim 1,wherein the photonic crystal structure includes defects formed bynanoimprinting a pattern and etching the pattern.
 8. A sensor apparatuscomprising: multiple sensor elements, each of the sensor elementsincluding a photonic crystal Mach Zehnder interferometer with areference arm and a sensor arm configured to provide an evanescent fieldthrough a sensed medium region adjacent to the sensor arm, each of thesensed medium regions being part of a photonic crystal structure of thephotonic crystal Mach Zehnder interferometer; and mechanism fordetecting outputs of the multiple sensor elements and converting theoutputs into data.
 9. The sensor apparatus of claim 8, wherein the meansfor detecting includes a photodetector.
 10. The sensor apparatus ofclaim 8, further comprising: means for processing the data to identifyarticles of media provided to the sensed medium regions.
 11. The sensorapparatus of claim 8, further comprising: means for transmitting thedata.
 12. The sensor apparatus of claim 8, further comprising: means fortransmitting the data in response to an interrogation signal received bythe sensor apparatus.
 13. The sensor apparatus of claim 12, wherein themeans for transmitting includes a radio-frequency identification (RE-ID)transponder.
 14. The sensor apparatus of claim 8, wherein one or more ofthe photonic crystal Mach Zehnder interferometers is formed from III-Vsemiconductor materials.
 15. The sensor apparatus of claim 8, whereinone or more of the photonic crystal Mach Zehnder interferometersincludes defects formed by nanoimprinting a pattern and etching thepattern.
 16. A method for sensing comprising: providing a photoniccrystal Mach Zehnder interferometer that generates an evanescent fieldthrough a sensed medium region which is part of a photonic crystalstructure of the photonic crystal Mach Zehnder interferometer; providingan article of medium at the sensed medium region; optically coupling alight source to an input of the photonic crystal Mach Zehnderinterferometer; and detecting an output of the photonic crystal MachZehnder interferometer.
 17. The method for sensing of claim 16, whereinthe article of medium is a chemical or biological medium.
 18. The methodfor sensing of claim 16, further comprising: processing datacorresponding to the output.
 19. The method for sensing of claim 16,further comprising: processing data corresponding to the output toidentify the article of medium.
 20. The method for sensing of claim 16,further comprising: transmitting data corresponding to the output. 21.The method for sensing of claim 16, further comprising: transmittingdata corresponding to the output in response to a sensor interrogationsignal.